Evaporation benefits served up on a plate

According to Alfa Laval Thermal, the first chemical plant to install one of the company's plate evaporators, in place of a conventional rising-film tube evaporator, has achieved significant benefits from making the switch. At its calcium chloride works in Helsingborg, Sweden, Kemira has improved capacity and reduced downtime for cleaning, while at the same time benefiting from cost and space savings of better than 50 per cent.

Traditionally, Kemira has concentrated its product in a multiple effect evaporation system using rising-film tube evaporators. The tube evaporator's life expectancy is limited, however, and Kemira had been planning to replace at least three of the effects over the next few years. Faced with a large capital investment for this, Kemira looked at alternative evaporator technologies and contacted Alfa Laval about its plate evaporator.

Originally developed for the sugar refining industry, this is a rising-film evaporator with a plate pattern developed specifically for evaporation duties. Its plate pack consists of a series of welded cassettes, in which conventional gasketted and laser-welded channels alternate. Evaporation takes place in the gasketted channel, while the heating vapour is confined to the welded one. The channel geometry promotes turbulence to enhance heat transfer, and reduces fouling.

Alfa Laval proposed replacing the third effect evaporator with a titanium plate evaporator. But, because plate evaporators were unproven on a base chemical process, Kemira demurred and asked for a pilot trial. Just three months into the trial, Kemira placed an order for a plate evaporator to replace the existing third effect.

'The unit has been in operation for more than six months now,' says Ove Uhrbom, Kemira's production manager, 'and is operating very well. There has been no need to clean it, but there has been a measurable increase in throughput despite the fact that, nominally, it has only half the heat transfer area of the tubular units.'